1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiplexing (OFDM) system. More particularly, the present invention relates to a method and apparatus for transmitting/receiving downlink data such that a User Equipment (UE) located in a Soft HandOver (SHO) region can combine data received from a plurality of cells.
2. Description of the Related Art
Recently, OFDM has widely been exploited for broadcasting and mobile communication systems. Due to the advantages of cancellation of interference between multi-path signals inherent to a radio channel, orthogonality among multiple access users, and efficient frequency use, OFDM out performs Code Division Multiple Access (CDMA) in application to high-speed data transmission and broadband systems. OFDM is a special case of MCM (Multi-Carrier Modulation) in which a serial symbol sequence is converted to parallel symbol sequences and modulated to mutually orthogonal subcarriers or subchannels, prior to transmission.
FIGS. 1A and 1B are block diagrams of a transmitter and a receiver, respectively in a typical OFDM system.
Referring to FIG. 1A, in the transmitter, a channel encoder 102 channel-encodes an input information bit stream. The channel encoder 102 may be a convolutional encoder, a turbo encoder or a Low Density Parity Check (LDPC) encoder. A modulator 104 modulates the coded stream by Quadrature Phase Shift Keying (QPSK), 8-ary PSK (8PSK), 16-ary Quadrature Amplitude Modulation (16QAM), or 64-ary QAM (64 QAM). While not shown, it is clearly to be understood to those skilled in the art that a rate matcher can be interposed between the channel encoder 102 and the modulator 104, for repetition and puncturing.
A Serial-to-Parallel (S/P) converter 106 parallelizes the modulated signal, and an Inverse Fast Fourier Transform (IFFT) processor 108 IFFT-processes the parallel signals. A Parallel-to-Serial (P/S) converter 110 serializes the IFFT signals (in other words an OFDM symbol). A Cyclic Prefix (CP) adder 112 adds a predetermined CP to the serial signal. A Radio Frequency (RF) transmitter 114 processes the CP-added signal by frequency upconversion, filtering and amplification and sends the resulting OFDM signal.
The OFDM receiver illustrated in FIG. 1B operates in the reverse order to the operation of the transmitter.
Referring to FIG. 1B, upon receipt of an OFDM signal at the OFDM receiver, an RF receiver 116 amplifies and downconverts the RF signal, and a CP remover 118 eliminates a CP from the OFDM signal received from the RF receiver 116. An S/P converter 120 parallelizes the CP-free signal, a Fast Fourier Transform (FFT) processor 122 FFT-processes the parallel signals, and a P/S converter 124 serializes the FFT signals.
A channel compensator 126 performs channel estimation and channel compensation on the serial signal. A demodulator 128 demodulates the channel-compensated signal in accordance with the modulation scheme used in the modulator 104 of the OFDM transmitter, that is, one of QPSK, 8PSK, 16QAM and 64QAM. A channel decoder 130 decodes the demodulated signal, thereby finally recovering an information bit stream. While not shown, it is to be clearly understood to those skilled in the art that a rate dematcher can be interposed between the demodulator 128 and the channel decoder 130, for performing the operation of the rate matcher in a reverse order.
For application of the OFDM system to a broadcasting or mobile communication system, aside from data, the transmitter must send a pilot signal and control information to enable demodulation and decoding of the data at the receiver. The pilot signal has a pattern known to both the transmitter and the receiver. When a signal is distorted during transmission on a radio fading channel, the receiver estimates the signal distortion and eliminates it using the pilot signal. The control information provides information about a modulation and channel coding scheme, a data block size, and Hybrid Automatic Repeat reQuest (HARQ) information such as a subpacket IDentifier (ID), used for the transmission data. The receiver demodulates and decodes the received data based on the control information.
Like other cellular communication systems, the OFDM cellular communication system requires a technique for improving the reception performance of a UE located at a cell boundary. For this purpose, the received Signal-to-Interference and Noise Ratio (SINR) of the UE is increased by reducing interference from neighbor cells. Each cell uses a randomizer for data transmission in order to reduce the neighbor cell interference. The randomizer is implemented using a scrambling sequence. The transmitter multiplies a transmission signal by a scrambling sequence specific to its cell, to thereby reduce the neighbor cell interference.
For improving the reception performance of a UE in an SHO region, i.e. a boundary of neighbor cells, besides the neighbor cell interference reduction, a technique for enabling the UE to combine signals received from multiple cells can be considered. Accordingly, there exists a need for developing a technique for enabling a UE at a cell boundary to combine and decode signals received from a plurality of cells.